This invention relates to instruments for measuring cathodic protection voltage levels and to the method used by such instruments to measure the voltage.
The oil and gas transmission industry uses cathodic protection on buried metal pipelines to minimize the amount of pipeline corrosion. Transmission pipe lines use a system which supplies a current through the pipe and a ground bed, causing the pipe to appear as a cathode of a chemical cell. The chemical action of the cathode is to take on particles. If the pipe were to be anodic rather than cathodic, the pipe would corrode and eventually cause a failure. Typical power supplies used to induce the cathodic protection voltage include a full-wave rectifier which outputs a signal derived from a commercial power line, generally 60 Hz AC. The resulting 120 Hz pulsating signal is applied between the pipeline and an associated ground bed in an attempt to maintain a suitable DC cathodic protection level. The applied voltage may or may not be filtered.
Test points are periodically provided along the pipeline to permit verification of the required cathodic voltage protection levels. Established standards provide that the level of cathodic protection is acceptable if it meets any one of at least five criteria. One of these criterias states that the pipeline should have a -0.85 V (or more negative) potential with reference to a copper-copper sulfate (Cu-CuSo.sub.4) probe. Such measurements are typically performed by using a high impedance average reading volt meter. Occasionally, inaccurate readings may be obtained because power line interference mixes with the desired signal measurement and the signal parameter being measured is not accurately measured due to noise induced on the ground path. Furthermore, the cathodic protection wave-form will often rise above the pipeline protection criteria for short periods during this 120 Hz wave-form. This condition, which is not detected using conventional average reading volt meters, may contribute to particle corrosion and should be detected.
This invention seeks to provide more accurate measurements of the cathodic protection voltage levels and to process the cathodic protection voltage level signals so that additional information, such as peak voltage or percentage of time that the voltage is above a threshold, may be obtained therefrom.